## Tuesday, October 27, 2015

### Techy Tuesday - Homemade Railgun

A friend at work sent me this link to a story about a guy who built his own railgun.  Railguns are popular research topics for the military, and use electric energy to accelerate the projectile.  Here, the builder used six large, 350V, 5500uF capacitors (in blue), which weigh 20lbs combined and can deliver 1800 Joules of energy to the projectile.  The gun is powered by a 12V lithium-polymer battery that’s stepped up to 1050V using a micro-inverter and a transformer before charging those capacitors.
Despite the fact that it doesn't appear to be painted or finished, it looks a lot like a gun from the first Men in Black movie.   The article is short on details, but says
The aluminum sabot flies out of the gun at 250 meters per second (559 mph), hits a plywood target with a steel backing approximately three feet away, and bounces off the wood after leaving a 1/2-inch deep indent
It doesn't say how much that sabot weighed, but we can look at what little information we have.  To begin with, 250 meters per second converts to 820 feet per second.  That's around the muzzle velocity of a 45 ACP handgun.  Obviously the impact energy depends on the mass being thrown, which they don't tell you.  A 12 ga. slug goes about 1600 fps and imparts almost 2600 ft-lbs at 25 yards (both of these courtesy of the Federal Premium online calculator)  That slug weighs about 1 1/4 ounce.  That 45 ACP round at 820 fps is going to be weigh a bit under 1/2 (42%) what the 12 ga. slug weighs and deliver a bit over 1/8 (14%) of the energy.

Dean Weingarten at AmmoLand reports he was able to find the mass of the railgun's sabot.  It's tiny.  Far, far less than the 230 grain FMJ I looked at on the ballistics calculator.
I was able to find the mass of the projectile with some Internet research.
...
The projectile mass was 1.1 grams, about 16.97 grains, so the total energy would be about 25 foot pounds.  That is about the same as the most powerful common air rifles, for the minimum velocity listed.  [Emphasis added - SiG]
For comparison, Federal's bulk 36 grain hollowpoint .22LR has a muzzle energy of 127 foot pounds.

From Crosman:
A powerful .22 caliber air rifle such as a Benjamin Discovery or Marauder fires a 14.3 grain pellet with a muzzle velocity of approximately 900 ft/sec. ... about 26 ft lbs.
Weingarten points out that this railgun is only operating at 1.6% efficiency in converting the electrical energy into forward movement, a big issue in railgun development.

Considering that this expensive, over 20 pound, rifle doesn't outperform a common .22LR out of a rifle, I'd say railguns have quite a way to go before they're practical.  I don't want to discourage the guy; the gun has an epic coolness factor, and as far as making a usable firearm goes, he's got to start somewhere!

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This is post #1900 here, and may well have to stay alone a few days.   I expect to be unavailable tomorrow and Thursday.  Y'all have fun!

1. I've often wondered if railgun coils could be used to boost the velocity of a conventionally-fired projectile.

I think, but haven't done the calculations, that a lot of the railgun's energy is burned in taking the projectile from a dead stop to final velocity.

Well, normal barreled-action firearms do a good job of getting a projectile up to speed, but boosting the speed electrically would increase muzzle energy by the SQUARE of the velocity increase.

2. Edit: s/final/low/

3. Interesting, but not very practical.

4. The 8086 processor, invented in 1982, executed 330,000 instructions per second. A modern Core i7 processor (not even top of the line) executes around 178 BILLION instructions per second. That means processor speed increased by a factor of 500,000 in just 30 years.

This rail gun is not even a prototype- it is more like a proof of concept. Where will it be in 20 years?

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6. And yet you can have the latest PC with an i7 and still find yourself sitting waiting for the damned thing to finish whatever it is it's doing. As our processor speeds have increased the demands on them have increased. My point here is most of those "demands" are not things that you the user/customer wants or needs but things that the manufacturers wanted and things that developers wanted and ultimately things that special interests and advertisers wanted. Next time you are waiting on your computer just understand it is spending billions of cycles doing someone elses bidding while you twiddle your thumbs.

7. I hate to break this to you, Anonymous on October 28, 2015 at 12:01 PM, but if you were to run something other than Microsoft or Apple, you might not have to wait so long. Ubuntu 64-bit seems to work rather well for me and a bunch of others, and there are those who swear by Linux Mint as well. There's a HUGE world out there beyond that of the left coast tech billionaires...

8. I've been running SuSE Linux since 1996.

Tried many others, but always came back to SuSE....

9. Do not assume that I am unaware of the alternatives. I choose to be part of the 95% for better or worse. My point is that the new CPU chips should have brought us world class computing power but it is being appropriated by the operating systems and associated software. As users we should demand better.

10. Just don't get too off topic. There are no processors in this thing, and nothing about the technology in it going to follow Moore's Law. It's more like batteries in electric cars: progress will be incremental and slow compared to make digital transistors smaller and faster.

11. Given the fact that most railguns are vehicle mounted not handheld makes this quite an achievement.

Given that early automobiles couldn't compete with a decent horse, doesn't mean that the people building cars were misguided. It just means that all those jackasses screaming "get a horse" had no vision.

And if this guy can do this with off-the-shelf stuff in his garage, do you thing DARPA might be able to do a bit more? In an age when there are actual hoverboards - granted they have some poor constraints as well - do you think there might be a future for railguns?